Hypodermic syringes, once contaminated, can be a source of dangerous and sometime fatal infectious diseases. Contaminated hypodermic needles, if not properly handled after use, can result in an unintentional needle stick of a health care worker. In addition, illicit use of used syringes can occur if the syringes are not properly disabled after use.
In response to some of these problems, a disposable, anti-needle-strike, anti-drug abuse syringe was developed and is disclosed in U.S. Pat. No. 4,170,170. The plunger on this syringe has a tapered opening which is used to engage a similarly tapered extension of the needle hub at the distal end of the barrel. This provides a frictional engagement which permits the user to rotate the needle hub, thus disconnecting it from the distal end of the barrel, by rotating the stem of the plunger. The frictional interface between the tapered opening and the tapered extension is designed to be sufficient to enable the user to pull the coupled needle assembly from the distal end of the barrel into the interior of the barrel shown in FIG. 14 in the patent. At this point the stem, which has a weakened region adjacent the piston, is snapped into two pieces leaving the distal end of the stem, the piston and the needle assembly inside the interior of the barrel. The hollow stem can then be inserted through the now open distal end of the barrel to cover the needle assembly and to permit compact disposal.
While this safety syringe provides significant advantages over existing safety syringes, the smooth, frustoconically shaped friction drive surfaces, being surfaces of revolution with respect to the syringe axis, proved less than ideal. One of the problems was the manufacturing tolerances for a good frictional fit between the frustoconical surfaces were quite close. Also, if moisture contaminated the tapered surfaces, especially if there was not good alignment between the two parts, the frictional engagement might not be sufficient to either disengage the barrel hub from the barrel or pull the barrel hub back into the barrel.